1. Field of This Invention
This invention relates to the prevention of color variation in a coating composition, such as, a paint, wherein color is produced by a mixture of pigments having different densities, caused by the separation of the pigments during storage or drying of the coating composition.
2. Prior Art
In preparing and applying mixtures of powdered or crystalline substances (such as, pigments) having different physical properties (such as, different specific gravity or density or different oil absorption) dispersed in fluid media (such as, oils or resin solutions or other film-forming paint vehicles), it is essential not only to accomplish a uniform dispersion of the mixed components, but also to obtain a state where such dispersed solid mixtures no longer have a tendency to separate from each other during storage of the dispersions. This is necessary because such pigment mixtures, comprising components of different specific gravity or different oil absorption, (after having been dispersed in the vehicle) tend to separate upon storage again into the various components. Therefore a pair consisting of pigments of different specific gravity will generally separate again with the heaviest component settling to the bottom of the paint and the less heavy components settling out there above. Shaking the stored mixture (in the vehicle) will not cause the separated materials to fully redisperse. Where such pigments have a different color, their initial dispersion in the vehicle represented a paint of a special color. But after separation of the components and after an incomplete remixing of the different pigments, the resulting product will not have the same color shade as when it had been produced initially. In paint production such a change in color shade due to these factors is highly undesirable.
Pigments having a similar specific gravity might vary in other physical properties which interfere with their uniform dispersion in fluid vehicles -- this will result in undesirable separations during storage of the dispersion. It might, for instance, be economically desirable to use a high priced colored pigment with the addition of a more economical colorless additive or "extender" pigment despite the fact that two such kinds of pigments might have widely different physical properties. For instance, a chemical pure zinc yellow might have a similar specific gravity as a wet ground mica extender pigment. But the oil absorption of the zinc yellow might be half that of the mica extender. In dispersing mixtures of both in an oil modified alkyd resin vehicle, the two pigments will widely differ in dispersion properties and in uniformity of application. The extender with its more incomplete dispersion in the oil based vehicle will settle out during storage and will not fully be redispersed with the other component under shaking or mixing. A different colorshade will be obtained and upon application a different reflectance will be obtained -- both are not desirable.
The literature and industrial practice have suggested various ways of improving the tendency of mixed pigments to separate in dispersions. For instance, a number of synthetic "surfactants" have been suggested for use in the dispersion of pigmentations, such as non-ionic derivatives of fatty acid esters (like polyethylene glycol 400 mono oleate, or non-ionic sodium salts of sulfonic acid or others). A wide group of other chemical synthetic additives have been suggested with the aim of relieving the storage difficulties of the paints having mixed pigmentations. But all these represent chemicals which differ in their chemical nature from that of the dispersing coating vehicle and none are known to have fully succeeded in overcoming the problem of the separation of the pigment components in such dispersions.
In order to improve the stability of pigment mixtures in paints, the prior art defines pigment separation as the result of three factors: floating of light weight color components to the paint surface ("FLOTATION"), a renewed agglomeration of pigment particles due to poor pigment wetting ("FLOCCULATION") and the settling out of pigments according to the Strokes Law ("SEDIMENTATION"). Kress, Peter, Deutsche Farben Zeitung 24, 11, p. 521 (1970), teaches the following:
__________________________________________________________________________ FORMS OF PIGMENT SEPARATION AND MEANS FOR LIMITATION TERM DEFINITION SUGGESTED MEANS OF LIMITATION __________________________________________________________________________ FLOTATION THE FLOATING OF LIGHT IMPROVE THE BONDING BETWEEN WEIGHT COLOR COMPONENTS PIGMENT SURFACE AND BINDER TO THE PAINT SURFACE BY A SOYALECITHIN (0.5 to 1.5% BASED ON THE VEHICLE.) FLOCCULATION RENEWED AGGLOMERATION REDUCE THE SURFACE TENSION OF PIGMENT DUE TO POOR OF THE BINDER BY A XYLENE PIGMENT WETTING. SOLUTION OF A SILICONE OIL (0.5 to 1% BASED ON THE TOTAL VEHICLE). SEDIMENTATION SETTLING OUT, ACCORDING RESTRICT THE FREEDOM OF TO STOKES' LAW. MOTION OF THE PIGMENT PARTICLES BY A BENTONE (2 TO 5% BASED ON THE VEHICLE. __________________________________________________________________________
By way of review, the freedom of motion of the pigment particles is supposed to be restricted by the addition of a certain amount of Bentone and the surface tension of the binder is supposed to be reduced by incorporation of a silicone oil. A third additive (0.5 to 1.5 percent soya lecithin, based on the vehicle) is introduced in order to "improve the bonding between pigment surface and binder". Since none of the three additives is volatile, each of them remains a component in the applied and dry final paint coating even though each physical role is no longer required.